Stress Corrosion Cracking

Introduction

A section of TP 304 stainless steel pipe used in a high pressure steam condensate return system experienced stress corrosion cracking. This pipe was installed outdoors, underground, beneath a sidewalk along a roadway. Interestingly, the cracking occurred near the welded ends, while the main body of the pipe remained intact. Understanding why this happened is crucial for stopping future failures.

Background

First, our client explained that the system’s pipes arrived with factory installed insulating jackets. However, they left a 1ft section at each end exposed for welding during installation. Then, after finishing the welding, technicians added additional insulating jackets in the field to protect the joints. This design choice seemed practical, but it may have introduced issues that were not seen.

Stress Corrosion Cracking Under Insulation
Figure 1. A length of TP 304 pipe suffered cracking in service as a high pressure steam condensate return. Left: the as received sample, weld at the far end. Right: after forcing open a main longitudinal crack.

Observation and Analysis

When we looked at the pipe sample, we noticed that the cracks were located at the seam between the factory installed and field installed jackets. This finding suggests that the seam likely added to the failure. It seems that the transition between these two insulating jackets created a weak point, leading to stress corrosion cracking under the operating conditions.

Figure 2. Upper left: Exterior surface wide shallow pitting along the main longitudinal crack. Evidence of chlorides was found in material taken from the damaged exterior surface (see Figure 3). Upper right: Interior surface with penetration pits and corrosion along the main longitudinal crack. Lower left: The main longitudinal crack face was corroded in stages suggesting penetration from the outside inward. Lower right: A longitudinal saw cut showed numerous feathery stress corrosion cracks coming from the exterior surface inward through the pipe wall.

Conclusion

In conclusion, the location and appearance of the cracks indicate that the seam between the factory installed and field installed jackets played a significant role in the failure. By finding this issue, we can recommend improvements in installation procedures and materials to prevent future failures.

Additionally, understanding how these seams affect the integrity of the pipe will help us develop better solutions.

Figure 3. Example SEM image and EDX spectrum of swabbed material taken from the pipe exterior surface. Note the presence of alloy corrosion products (Fe, Cr, Ni) and additional elements including Cl. Chlorides, for example, from road salts, can speed up corrosion of stainless steel and promote localized attack.
Figure 4. Mounted, polished, and etched surfaces of cracked (left) and healthy (right) metal samples are shown. The microstructure is as expected for an austenitic stainless steel 304 in both cases. The crack path, especially of the finer cracks, was primarily transgranular rather than intergranular. In combination with other results, this favors stress corrosion cracking over other possible mechanisms such as sensitization, which can occur near welds in the presence of chlorides but is primarily intergranular in its crack propagation.

Contact Us

If you want more information about stress corrosion cracking or want to discuss how we can help with your material evaluation needs, please contact us at Anderson Materials Evaluation. Our team of experts is ready to provide insights and solutions tailored to your specific requirements.